DE69224365T2 - Process for coating printed circuit boards - Google Patents

Abstract

A dual curing composition is prepared from components consisting essential of (a) an isocyanate adduct having (i) free isocyanate groups and (ii) free photopolymerizable ethylenically unsaturated groups, wherein the ethylenically unsaturated groups comprise from 10 to 70% of the total of any free functional groups on said adduct; (b) a reactive (meth)acrylate diluent; and (c) optionally, photoinitiator. When cured, the above composition is particularly suitable as a conformal coating on electronic circuit boards.

Description

This invention relates generally to a method of providing a conformal coating on an article using dual-cure conformal coatings. By "conformal coating" is meant a coating that is applied to conform to the characteristics of the surface to be coated, particularly electronic circuit board surfaces. By "dual-cure" is meant that the curing occurs by two mechanisms.

Conformal coatings were originally made to protect electronic components for military and aerospace applications from environmental factors such as moisture, dust and chemical contamination. These environmental factors cause unprotected components with sensitive circuits to malfunction due to corrosion and electrical shorts. As electronic packages become denser and have finer pitches between traces, the components are even more sensitive to environmental factors. In addition, the increasing use of electronic components in transportation and general industrial applications is causing the increasing use of conformal coatings, primarily to protect the electronic components from the aggressive environments of their end use.

The most damaging and most common contaminant is generally considered to be moisture or humidity. Excessive moisture or humidity reduces the dielectric resistance between conductors, accelerates high voltage breakdown and dendrite growth, and corrodes circuit traces. Other contaminants that can damage printed circuit boards include various chemicals such as residues from the manufacturing process, e.g. flux, organic solvents, release agents, metal particles and marking inks, and contaminants that may be inadvertently introduced by human handling, such as body oils, fingerprints, cosmetics and food stains. The working environment can also contribute to a variety of contaminants such as salt spray, dirt and dust, oil, fuel, acid, corrosive fumes and fungi. In all but the most severe cases, the destructive effect of these contaminants can be effectively eliminated by providing a good conformal coating.

In addition to providing protection against contamination, conformal coatings also provide a certain degree of protection against mechanical impact (shock), vibration and tampering.

In the past, the first engineered conformal coatings were one-part and two-part systems whose main components were acrylic, epoxy, silicone or polyurethane resins. These resins were usually dissolved in solvents so that they could be applied to the parts by spraying, dipping or brushing. These first generation coatings (coatings) had one or more disadvantages, such as the need for mixing before use and the use of large amounts of solvent to reduce viscosity for application. They also typically required long drying/curing times which released large amounts of volatile organic compounds (VOCs) before the coated parts could be handled.

Some of these disadvantages were addressed by the introduction of ultraviolet (UV)-curable conformal coatings. The UV-curable coatings were generally solvent-free, one-part systems that cured via free radical polymerization upon exposure to UV light, so they were non-sticky to the touch. This allowed immediate handling of the coated parts, thus reducing processing time and energy costs compared to thermally cured solvent-based coatings. The application viscosity of this first generation of UV-curable coatings tended to be higher than that of solvent-based systems. Although they were solvent-free, the viscosity could not be easily adjusted, making the application of thin films more difficult.

The rapid conversion from a liquid composition to a cross-linked solid occurred only in the areas exposed to UV light and not in the masked areas (e.g., under components). This shadowing became a more serious problem as higher component density electronics became more common. Therefore, a second curing mechanism such as heat was sometimes incorporated into UV-curable conformal acrylic coatings to cure these masked areas while maintaining the stability of one-part systems. However, thermal curing regimes of ≥ 100ºC were required to complete this second curing process.

Various dual-curing, conformal coating systems are known in the art, each with its advantages and disadvantages.

For example, dual-cure two-component systems such as two-component polyurethane systems offer short cure times, see US-A-4,424,252 to Nativi et al. In particular, US-A-4,424,252 to Nativi et al. discloses a two-component system comprising reacting a urethane acrylate with an aliphatic or aromatic polyisocyanate adduct. The coating then cures within 2 hours to one day by several mechanisms, one of which involves the reaction between free isocyanates on the polyisocyanate adduct and atmospheric moisture. The curing mechanism also involves the reaction between the isocyanate groups and the hydroxyl groups on the urethane acrylate. As a result of the latter reaction, this system, as well as the other two-component systems, see e.g. B. US-A-5 013 631 to Su, has a limited pot life (up to about 48 hours or less) and thus forces the user to perform the mixing step immediately before use.

One-component systems avoid the pot life problems associated with two-component systems by avoiding the inclusion of any isocyanate-reactive functional groups into the system, see US-A-4,415,604 to Nativi. For example, US-A-4,415,604 to Nativi discloses a one-component system comprising an isocyanate-capped polyether diol and triol, an acrylate diluent, and a photoinitiator. However, because the isocyanate-capped diols and triols are based on hydrophilic polyethers, the resulting coating loses some of its hydrolysis resistance and moisture resistance. Thus, in more humid environments, the coating is less effective.

Another type of one-component dual-cure conformal coating involves a curing mechanism other than an isocyanate reaction. For example, U.S. Patent No. 4,451,523 to Nativi et al. discloses a one-component composition comprising a urethane (meth)acrylate, an allyl group-containing (meth)acrylate monomer, a non-allylic (meth)acrylate diluent, photoinitiator, and a metal drying agent. Crosslinking of the allylic compounds in the presence of metal drying agents provides the second curing mechanism in addition to the UV curing of a (meth)acrylate. However, this system can contribute metal ionic species that degrade the electrical properties of the coating. These species can also cause aging of electronic components by promoting electrical path formation between closely packed components.

It is also known to use dual cure coatings in fields other than electronic components. For example, radiation dual curable coatings have been used in the automotive industry, see US-A-4 173 682 to Noomen et al. In particular, Noomen et al. disclose a two-component automotive coating system comprising (i) an isocyanate group-containing adduct prepared from a (meth)acrylic hydroxy ester and a polyisocyanate, and (ii) a polyfunctional hydroxy compound. However, this two-component system not only suffers from the disadvantages discussed above for electronic circuits, but it also appears to suffer from long cure times at ambient conditions, e.g. several days, compared to cure times for other two-component systems. In addition, some of the isocyanates disclosed by Noomen et al., e.g. the adduct of hexamethylene diisocyanate and water, contain biuret bonds. Compounds containing these bonds are not among the most heat-resistant or "weather-resistant". By "weather-resistant" is meant resistance to moderate humidity, temperature fluctuations and sunlight.

US-A-4,138,299 to Bolgiano discloses a one-component dual-cure composition suitable for glossy abrasion-resistant floor coatings. Specifically, Bolgiano discloses a one-component composition consisting essentially of an isocyanate-terminated (NCO) prepolymer (prepared from an aliphatic isocyanate and a polyester diol and triol) and an acrylate diluent containing no reactive hydroxyl groups. The NCO-terminated prepolymer is also further reacted with sufficient hydroxyacrylate to cap 5 to 15% of the available NCO groups. The composition cures primarily by crosslinking the ethylenically unsaturated groups of the acrylates and to a lesser extent by chain extension and crosslinking of the free NCO groups and water. As with the composition disclosed in US-A-4,415,604 to Nativi, Bolgiano's composition contains hydrophilic moieties, i.e. the polyester bonds. As a result, in humid environments the hydrolytic stability of the composition is not as effective.

EP-A-0 126 359 discloses a process for preparing compounds which crosslink under high energy irradiation and contain isocyanurate groups, olefinic double bonds and blocked or free isocyanate groups.

As is apparent from the above discussion and well known in the art, there is no dual-cure conformal coating that is completely satisfactory for all applications due to the different processing (e.g., processing speed, pot life and curing conditions) and end-use requirements (e.g., thermal characteristics, weatherability and hydrolytic resistance). Accordingly, The intended application and the environment associated with the application typically dictated the specific formulation of the coating.

The present invention utilizes a dual UV/moisture curable conformal coating composition which is a solvent-free, one-part, low viscosity, storage stable liquid that rapidly cures to a non-tacky solid upon exposure to UV light. This UV curing allows for immediate handling of the coated part and prevents runoff of the liquid coating trapped in the masked areas. The coating in the masked areas polymerizes to a touch-dry solid upon exposure to air by reacting with atmospheric moisture.

An additional advantage of the present invention is that it is adaptable to different process requirements. For example, it does not mind high humidity conditions. That is, its static pot life tolerates, to some extent, moderate humidity and heat in the production environment. UV curing does not have to be complete to produce the tack-free surface. Instead, exposure only needs to be sufficient to initiate curing. The initial surface tackiness is eliminated upon standing. It is thus possible to reduce the duration and/or intensity of UV exposure to the articles if desired. The longer curing time and low viscosity of the liquid can contribute to a reduction in defects in the products. Carbon dioxide can form during the curing stage as a by-product of the polymerization reaction. If the coating cures somewhat more slowly and the initial viscosity is low, this gas has more time to diffuse out of the coating rather than forming bubbles. Additionally, there is more time to inspect and repair defects in the components during manufacturing. Finally, a thinner, more uniform coating can be obtained.

The present invention provides a method for creating a conformal coating on an article having covered areas, in which

(a) a one-part, dual-cure, conformal coating composition is applied to the article,

(b) the coating is exposed to radiation and

(c) allowing atmospheric moisture to complete the curing of the coating,

wherein the coating composition is solvent-free and

(i) an isocyanate adduct having free isocyanate groups and free photopolymerizable ethylenically unsaturated groups, the ethylenically unsaturated groups accounting for 10 to 70% of the sum of all free functional groups of the adduct,

(ii) a reactive (meth)acrylate diluent and

(iii) optionally photoinitiator

includes.

The invention further provides a method for creating a contour-accurate coating on an object having shadow areas, in which

(a) a one-part, dual-cure, conformal coating composition is applied to the article,

(b) the coating is exposed to radiation and

(c) allowing atmospheric moisture to complete the curing of the coating,

wherein the coating composition is solvent-free and comprises aliphatic isocyanate trimer having an isocyanurate ring.

The method of the invention can be used not only as a coating on electronic articles, but also on other articles. Specifically, a composition as described above is applied to the article and then exposed to radiation to provide the primary cure. A second cure results from the reaction of the free isocyanate groups of the composition and water.

As shown above, the composition used according to the invention is a one-component composition. With this By this term it is meant that the composition is supplied to the user as one formulation suitable for immediate use and is relatively stable on storage, e.g., at least thirty days at room temperature. As a result, the one-part composition should consist essentially of (i) through (iii) and should be substantially free of any free isocyanate-reactive functional groups, e.g., hydroxyl groups. As previously discussed, two-part systems are supplied to the user as two formulations which require a dispensing and mixing step prior to use. Such compositions are generally not stable on storage once mixed.

The isocyanate adduct referred to above as (i) is the reaction product of a polyisocyanate and a hydroxyalkyl (meth)acrylate. As a result of this reaction, (i) not only has free isocyanate groups, but also has free ethylenically unsaturated groups.

Suitable polyisocyanates are well known in the art. It is preferred that the polyisocyanate have 2 to 3 free isocyanate groups. For applications where the appearance of the coating is not critical, suitable aromatic polyisocyanates include toluene diisocyanate, diphenylmethane-4,4'-diisocyanate, naphthalene diisocyanate, 3,3'-bistoluene diisocyanate, and 5,5'-dimethyldiphenylmethane-4,4'-diisocyanate. Adducts of the above isocyanates are also suitable.

In applications where the appearance of the coating is critical, aliphatic polyisocyanates are preferred. Suitable aliphatic isocyanates include tetramethylene 1,4-diisocyanate, hexamethylene 1,6-diisocyanate, ω,ω-dipropyl ether diisocyanate, thiodipropyl diisocyanate, 1,2,4-trimethylhexane 1,6-diisocyanate, 2,4,4-trimethylhexane 1,6-diisocyanate, cyclohexyl 1,4-diisocyanate, isophorone diisocyanate, the adduct of one molecule of 1,4-butanediol and two molecules of isophorone diisocyanate, the adduct of one molecule of 1,4-butanediol and two molecules of hexamethylene diisocyanate, dicyclohexylmethane 4,4'-diisocyanate, dicyclohexyldimethylmethane 4,4'-diisocyanate, xylene diisocyanate, 1,5-dimethyl(2,4-ω-diisocyanatomethyl)benzene, 1,5-Dimethyl(2,4-107-diisocyanatoethyl)benzene, 1,3,5-trimethyl(2,4-ω-diisocyanatomethyl)benzene and 1,3,5-triethyl(2,4-ω-diisocyanatomethyl)benzene.

A preferred aliphatic polyisocyanate is an aliphatic isocyanate trimer having an isocyanurate ring. The trimer is preferably the trimerization product of hexamethylene diisocyanate having the following structure:

The above polyisocyanate is commercially available as DESMODUR N-3300 polyisocyanate from Mobay.

Other suitable aliphatic isocyanate trimers include those prepared from isophorone diisocyanate and dicyclohexylmethane diisocyanate.

The aliphatic isocyanate trimers are preferred because conformal coatings made from them are more heat and weather resistant than polyisocyanates containing biuret linkages, e.g., the polyisocyanate above made from hexamethylene diisocyanate and water. See U.S. Patent No. 4,173,682 to Noomen et al. Indeed, such trimers would be useful not only for conformal coatings consisting essentially of (a) through (c), such as those above, but also for other conformal coatings.

The hydroxyalkyl (meth)acrylates suitable for reaction with the polyisocyanate include, but are not necessarily limited to, hydroxyethyl (meth)acrylate, 2-hydroxymethyl (meth)acrylate, hydroxybutyl (meth)acrylate (and isomers thereof), and 2-hydroxypropyl (meth)acrylate (and isomers thereof).

The formation of an adduct from a polyisocyanate and a hydroxy(meth)acrylate can generally be carried out by combining the reaction components in an arbitrary manner, optionally at elevated temperature. it is preferred that the reaction takes place under anhydrous conditions at temperatures in the range of 5°C to 100°C, especially in the range of 50°C to 90°C. In general, the reaction components can be combined in any chosen manner. However, if desired, the hydroxyalkyl (meth)acrylate can be added to the polyisocyanate in several stages. The reaction is usually carried out in the presence of an inert solvent such as methyl isobutyl ketone, toluene, xylene or esters such as butyl acetate or ethyl glycol acetate, but the use of a solvent is not absolutely necessary. Optionally, the reaction between the isocyanate groups and the hydroxy groups can be carried out in the presence of a catalyst. Suitable examples of catalysts include tertiary amines and organic tin salts or zinc salts such as dibutyltin dilaurate, tin octanoate and zinc octanoate. Mixtures of catalysts can also be used.

As stated above, the amount of hydroxyalkyl methacrylate is added and the reaction is carried out for a sufficient time to ensure that at least 10%, but not more than 70%, of the free functional groups on (a) are ethylenically unsaturated groups, with preferred amounts being in the range of 20 to 40%. In other words, the amount of hydroxyalkyl (meth)acrylate and the reaction time should be sufficient to terminate at least 10% of the free isocyanate groups on the polyisocyanate, but not more than 70%, preferably 20 to 40% of the isocyanate groups are terminated.

Once the polyisocyanate adduct has been prepared, it is mixed with at least one and preferably two or more reactive (meth)acrylate diluents. Suitable diluents include those well known in the art and examples thereof, but not necessarily limited to, are isobornyl acrylate, isodecyl acrylate, phenoxyethyl acrylate, dicyclopentenyl ethoxy methacrylate and dicyclopentenyl ethoxy acrylate. Others include trimethylolpropane ethoxylate triacrylate, dicyclopentadiene methacrylate and Dicyclopentadiene acrylate, isobutyl methacrylate and lauryl methacrylate.

The mixture may include additives well known in the art. For example, a photoinitiator or a mixture of two or more photoinitiators or photocure rate accelerators may be added. Suitable photoinitiators include, but are not necessarily limited to, ultraviolet (hereinafter "UV") activated free radical generators and may typically be used in amounts of from about 1% to about 10% by weight of the coating composition. For example, the UV activated initiators can be selected from metal carbonyls having the formula Mx(CO)y, where M is a metal atom, x is 1 or 2 and y is an integer determined by the total valence of the metal atoms, generally 4 to 10. The preferred UV activated initiators are selected from (a) straight chain or branched chain C1-16 alkyldiones and (b) carbonyl compounds having the general formula R5(CO)R6, where R5 is a C1-10 alkyl, aryl, aralkyl or alkaryl group and R6 is R5 or hydrogen. Additionally, R5 or R6 can be contain any substituents which do not adversely affect the compound in performing its function. For example, R⁵ or R⁶ may be α-substituted with an alkyl, aryl, alkaryl, alkoxy or aryloxy group, or with an amino or mono- or dialkylamino derivative thereof, wherein each of the above substituents contains up to about 6 carbon atoms. In addition, R⁵ and R⁶ may form together with the carbonyl group(s) an aromatic or heterocyclic ketone containing up to about 16 carbon atoms.

Specific examples include benzophenone, o-methoxybenzophenone, acetophenone, o-methoxyacetophenone, acenaphthenoquinone, methyl ethyl ketone, valerophenone, hexanophenone, α-phenyobutyrophenone, p-morpholinopropionphenone, dibenzosuberone, 4-morpholinobenzophenone, benzoin, benzoin methyl ether, 4-o-morpholinodeoxybenzoin, p-diacetylbenzene, 4-aminobenzophenone, 4'-methoxyacetophenone, α-tetralone, 9-acetylphenanthrene, 2-acetylphenanthrene, 10-thioxanthenone, 3-acetylphenanthrene, 3-acetylindole, 9-fluorenone, 1-indanone, 1,3,5-triacetylbenzene, thioxanthen-9-one, xanthen-9-one, 7-H-benz[de]anthracen-7-one, benzoin tetrahydropyranyl ether, 4,4'-bis(dimethylamino)benzophenone, 1'-acetonaphtone, 2'-acetonaphtone, acetonaphthone and 2,3-butanedione, benz[a]anthracene-7,12-dione, 2,2-dimethoxy-2-phenylacetophenone, α,α-diethoxyacetophenone, α,α-dibutoxyacetophenone, etc. Singlet oxygen generating photosensitizers such as rose bengal, methylene blue and tetraphenylporphine can also be used as photoinitiators. Polymeric initiators include poly(ethylene carbon monoxide) and oligo(2-hydroxy-2-methyl-1-[4-(1-methylvinyl)phenyl]propanone]. The use of a photoinitiator is preferred because it generally provides faster and more efficient initiation. When actinic radiation is used, the initiators can also provide initiation at longer wavelengths that are less expensive to produce and less hazardous.

It should be noted, however, that when energy sources other than visible or ultraviolet light, e.g., ionizing radiation, are used to initiate the curing reaction, photocuring rate accelerators (i.e., photosensitizers, etc.) are not necessarily required in the formulation. For practical reasons, these materials are usually necessary to produce an economically viable process.

It is useful, but also not necessary, to include an adhesion promoter in the composition when it is used as a coating or as an adhesive. Suitable adhesion promoters include those known in the art. Suitable promoters include those known in the art. Suitable promoters include mono- or dicarboxylic acids capable of copolymerizing with the acrylate or methacrylate reactive diluents, well-known silane promoters, organic titanates, and organic zirconates. The amount of promoter varies depending on the type used. Suitable amounts may range from 0.01 to 20% of the composition.

Also optional is the addition of one or more chelating agents, crosslinking agents and/or polymerization inhibitors. Chelating agents and inhibitors are effective in amounts of about 0.1 to about 1 weight percent of the total composition. Ethylenediaminetetraacetic acid and its sodium salt and β-diketones are generally most effective and are preferred. Crosslinking agents may be present in amounts of about 0 to about 10 weight percent of the total composition and include compounds such as copolymerizable di(meth)acrylates.

The inhibitor concentration left over in the monomers from manufacture is often high enough for good stability. To ensure maximum pot life, the proportions mentioned (about 0.1 to about 1% by weight of the composition) are recommended. Suitable inhibitors include the group consisting of hydroquinones, benzoquinones, naphthoquinones, phenanthraquinones, anthraquinones, and substituted compounds of any of the foregoing. Various phenols can also be used as inhibitors, the preferred being p-methoxyphenol. In addition, alkyl or aryl phosphites can be used, with triphenyl phosphite and triisodecyl phosphite being preferred.

The thickness, viscosity or thixotropy of the composition may be varied according to the particular application required. Thickeners, plasticizers, diluents and various other reagents conventional in the art may be used in any reasonable manner to produce the desired characteristics.

It is also optional that surfactants may be present in the coating system for optimum performance. Suitable surfactants are those well known in the art that are soluble in the coating and unreactive with the isocyanate-capped composition. Suitable surfactants include siloxanes and fluorocarbons. Other suitable surfactants include anionic materials such as petroleum sulfonates, sodium alkyl or alkylaryl sulfonates, and sulfonated ethoxylated types.

The surfactant concentration depends on the particular surfactant and reactive diluent used. However, a minimum concentration of about 0.25% surfactant, by weight of the composition, is usually necessary and a concentration of at least about 0.5% is usually preferred. The maximum surfactant concentration is usually about 10%, since above this level the surfactant begins to interfere with the properties of the coating and encapsulation system, for example, by adversely affecting its cure rate, durability or the cured products. In practice, an upper concentration limit of about 5% is usually satisfactory. For most surfactants or combinations of surfactants, the optimum concentration probably falls in the range of about 1.0 to 2.5% by weight of the total composition.

Components (i) to (iii) and any optional additives may be mixed together simultaneously or the adduct (i) may be mixed with the other components using conventional masterbatch techniques.

Once the desired components have been formulated, the one-part composition can be applied to articles using conventional dipping, spraying, shower coating, pour coating and brushing techniques.

Curing is believed to be best accomplished by UV activation of the photoinitiator(s) which break down into free radicals which initiate polymerization of the reactive diluents and the acrylate portion of the oligomer. A high intensity UV radiation source such as a medium pressure mercury arc lamp is the preferred UV light source. These high intensity UV light sources are preferred because they have sufficient intensity to overcome the various inhibitory effects found in free radical polymerized systems. These include the stabilizers/inhibitors added to improve storage stability, but these tend to retard bulk cure. The other major inhibitory effect is caused by oxygen in the air, which acts as a free radical scavenger, retarding surface cure, particularly in thin acrylate-based coatings. The conformal coating produced by the present invention overcomes any cure problem of tack-free surface, such as in the partially masked areas around components, apparently due to the unique nature of the oligomer used, continuous cure initiated by the UV treatment, and its secondary moisture cure. Any minor surface tack should disappear at ≥ 50% humidity within 24 to 48 hours.

This prolonged cure also polymerizes any liquid coating in the masked areas such as under components and protrusions that remain after the initial cure.

The liquid coating material is somewhat "forgiving" of the moisture exposure that is sometimes present in a production environment. Examples of static pot life of the coating are illustrated in Example 8, where the coating was exposed to "standard" temperature conditions and slightly elevated temperatures.

It was found that the static pot life of the liquid coating material was most affected by high surface to volume ratios in the reservoir. This was to be expected since atmospheric moisture diffuses from the exposed surface into the bulk of the coatings. However, even at a surface to volume ratio of 1:3, the liquid material was still usable after 7 days at room temperature and 50% relative humidity. Technical dip tanks typically have surface to volume ratios greater than 1:12, so one would expect an even longer pot life. An indication of this longer pot life is suggested by the fact that the 1:8 reservoir maintained at 35ºC/32% relative humidity had a usable pot life in the reservoir of > 12 days.

As with the other isocyanate-containing, double-curing compositions, the composition used according to the invention cured by two mechanisms. The first cure is obtained by exposing the coating to radiation, preferably UV radiation, to effect crosslinking of the free ethylenically unsaturated groups. By exposure to moisture, preferably atmospheric moisture, crosslinking and chain extension of the free isocyanate groups in the composition takes place. In this way, the irradiated areas of the coating become "dry to the touch", allowing immediate handling of the coated article and maintaining the shape of the coating which might otherwise run and creep. The second cure provides substantially complete cure of the non-irradiated areas of the coating under conditions of moderate temperature and humidity.

When the composition is used as a conformal coating on electronic circuit boards to which various components are mounted, the "dry to the touch" cure is important because it allows the coated board to be processed fairly quickly after it is coated. The second cure provides a mechanism for curing the areas of the coating that were protected from irradiation by the mounted components. Articles with surface features that cause similar shadowing problems also benefit.

Alternatively, a "dry to the touch" cure is not required to ultimately produce a tack-free surface. Acrylate coatings that are UV curable tend to retain a tacky surface if they are not fully cured. The coating of the invention can be partially cured by exposure to UV. Any slight surface tack will disappear within about 24 to 48 hours. This feature can provide a manufacturer with a cost savings because the time or intensity of UV treatment can be reduced.

The coating according to the invention also shows very good physical properties such as heat, weathering and hydrolysis stability, see Example 6. These features are particularly important for contour-accurate coatings for electronic circuit boards, which, as previously stated, are continuously exposed to solvents, fluxes, contact, etc. These properties are also suitable for other applications of the invention, including potting compounds and adhesives.

To further illustrate the practice of the present invention, the following examples are given, but are not intended to be limiting in any way.

example 1 Synthesis of polyisocyanate adduct Footnotes:

Polyisocyanate (CAS 28182-81-20) from Mobay; based on 5.2 mEq isocyanate (NCO)/g by titration (21.8% NCO).

b ROCRYL 420 hydroxyethyl acrylate (HEA) (CAS 818-6-1) from Rohm & Haas. Based on 8.6 meq hydroxyl (OH)/g using 20% of the stoichiometric amount. Dried over 4 Å molecular sieves before use.

625 g of DESMODUR N-3300 polyisocyanate was added to a dried 1 liter resin vessel equipped with a glass and Teflon paddle stirrer, stainless steel clad temperature probe, additional pressure equalizing dropping funnel and dry air blanket. The polyisocyanate was introduced in such a manner as to minimize contact with atmospheric moisture. The polyisocyanate was heated to 45 to 50ºC (113 to 122ºF) while stirring at 200 rpm, at which time 3.5 g of triphenyl phosphite and 0.4 g of p-methoxyphenol were added to the vessel. The mixture was stirred for 5 minutes while allowing the vessel temperature to rise to 55ºC (131ºF). The stirrer was accelerated to 400 rpm and 75.6 g of HEA was then added dropwise over the addition funnel to the reaction mixture over 15 minutes (average addition rate approximately 5 g/min). The reaction mixture was allowed to heat from 55ºC (131ºF) to 65ºC (149ºF) by the exotherm during the HEA addition. After the HEA addition, the mixture temperature was allowed to rise to 85-90ºC (185-194ºF) by the exotherm and additional heating. The mixture was then further stirred at 85-90ºC (185-194ºF) until the isocyanate value stabilized at 3.65-3.75 meq/g as measured by conventional titration techniques.

The resulting product was a clear to very slightly turbid, colorless, somewhat viscous liquid at room temperature. The typical Brookfield RVT viscosity using a #14 spindle and SC4-6R sample chamber at 25ºC (77ºF) was 11,500 centipoise (cps).

Example 2 Contour-accurate coating formulations

Two coating formulations A and B were formulated based on the polyisocyanate/HEA adduct formed in Example 1 using masterbatches of (meth)acrylate monomers, photoinitiator and stabilizer. The materials and amounts of monomer masterbatches and coatings A and B prepared include the following: Monomer masterbatches

Footnotes:

Sartomer Co. b Röhm & Haas c Ciba Geigy Covers

The monomers and MEHQ were placed in an amber glass bottle and then shaken together until the MEHQ was dissolved. The Irgacure 651 photoinitiator was added and the mixture was shaken for an additional 5 minutes until the stabilizer and photoinitiator were dissolved. The monomer masterbatches were then dried by adding 40 g of 4 Å molecular sieves (8 to 12 mesh) to each and then allowed to stand in the sealed bottles for approximately 4 to 6 hours.

Two coatings were then formulated in a dry box (< 15% relative humidity) to minimize contact with moisture. Contact with air was kept to a minimum. The adduct prepared according to Example 1 was then placed in a dry amber glass bottle or plastic bottle, followed by the properly dried monomer masterbatch. The mixture was shaken at room temperature for five minutes until the oligomer was dissolved.

Both coatings (A and B) were transparent, essentially colorless liquids. The Brookfield viscosities of A and B were 115 cps each. After several weeks of storage in sealed containers, the viscosities were approximately 125 to 130 cps, indicating that the formulations were storage stable.

Example 3 Hardened coatings

Printed circuit boards (PCBs) made from 28.4 g copper and 1.6 mm core FR-4 with surface mounted "dummy" devices (SMD) attached with tin-lead solder were cleaned using procedures common in the electronics field. The PCBs were then dip coated with coatings A and B from Example 2 by immersion in a coating bath at a constant rate of 25.4 cm/min and then removing the coated part at about 10 cm/min.

The coated part was cured by exposure to UV radiation until the coating was dry to the touch. The exposure technique involved exposing each of the PCBs to the UV light using a Colight curing system (2 x 200 watt/inch Hg lamps, belt speed 30 ft/min). Each side (top and bottom) received a total of 3 passes. The coating under the SMDs remained liquid while the exposed areas were tack-free. These coated UV-cured parts were then placed in a 100% relative humidity chamber for 48 hours and then placed in an open room at 30% relative humidity for 13 days, after which one of the SMDs was removed on the "A" and "B" coated boards. In each case, the coating under the SMD part was tack-free.

Example 4

Approximately 0.8 g of each of coatings A and B from Example 2 in aluminum weighing pans were placed in chambers maintained at 50% relative humidity and 100% relative humidity. After 72 hours in the 100% humidity chamber, both coatings were tack-free. The samples maintained at 50% relative humidity for 144 hours were also tack-free.

Example 5 Accelerated moisture curing

Accelerated moisture curing was carried out by adding 0.15 g of a 1 wt% solution of dibutyltin dilaurate in isodecyl acrylate to 15 g of each of coatings A and B. The moisture cures of these mixtures were tested in the same manner as in Example 4. Both of these mixtures containing the dibutyltin dilaurate were clear solids after 24 hours at 50% and 100% relative humidity and were tack-free within 48 to 72 hours. When moisture curing was tested at 30% relative humidity, both of these mixtures were tack-free after 48 hours, while the coatings without the tin compound remained liquid. After 30 days of storage in sealed plastic bottles, the viscosity of this composition increased from 125 cps to 140 cps at 25°C.

Example 6 Properties of the hardened coating

Tests for the following properties were conducted on cured conformal coatings prepared according to Example 2 and cured by a procedure similar to that described in Example 3, except that the procedure included techniques in accordance with MIL-I-46058C. Specifically, the coated test coupons were dip coated at a dip and withdrawal rate of 10 cm/min and UV cured using 4 passes per side. After the coupons were cured, they were placed in a humidity chamber for 72 hours. (100% relative humidity). They were then removed and placed in an open room for the last 24 hours before testing. Both A and B passed the tests.

(1) Chemical resistance: Measured by rubbing back and forth (1 double rub) with a cotton pad soaked in methyl ethyl ketone over the coated sample. The same test was repeated using a pad soaked in a mixture of Freon and isopropyl alcohol. Both A and B survived 100 double rubs with each solvent.

(2) Thermal shock: as substantially measured by military specification MIL-I-46058C. Both A and B withstood fifty cycles of alternating exposure to -55ºC and 125ºC, with 17 cycles being -55ºC to 125ºC and 33 cycles being -65ºC to 125ºC.

(3) Humidity resistance: measured by military specification MIL-I-46058C. Both A and B show dielectric resistance measurements of at least or greater than 10¹⁰ Ω before, during and after testing, which included examining resistance under electrical load and under high humidity (90 to 95% relative humidity) and under temperatures of 25º to 65ºC.

(4) Dielectric strength: measured by military specification MIL-I-46058C. Both A and B passed this test by having no breakdown discharge under the test voltage of 1500 volts AC at 60 Hz and with a current leakage of less than 10 microamperes.

(5) Insulation resistance: measured by military specification MIL-I-46058C, which was previously used as insulation resistance before the moisture resistance test described in (3) above. Both A and B passed with an insulation resistance of 2.0 x 10¹⁴ Ω.

Example 7 Alternative hardening process

Thin films of the invention have the ability to cure to a tack-free surface after a partial ultraviolet (UV) cure which leaves the surface of the coating still tacky. It is believed that this ability of the tacky surface of the coating to become dry to the touch is made possible by continuing the cure initiated by the UV treatment or by a secondary moisture cure mechanism as described in Examples 3 and 4.

Formulation B from Example 2 was drawn down on the Hegman gauge with a 0 to 25 um deep groove (about 0.5" wide x about 6" long) so that the groove was filled with the formulation. The gauge was passed through a Colight UV curing system with 4 passes at 30 ft/min. The coating was tack-free in the 25 um to about ~10 um area of the Hegman gauge. The ~um to 0 um thick areas of the coating remained tacky. The gauge with the UV irradiated coating was placed in a 95 to 100% relative humidity chamber for 24 hours, after which time the area of the coating that was tacky after UV irradiation was tack-free to the touch.

Example 8 Static pot life

Composition "B" from Example 2 was placed in the following plastic containers, all of which were open (no lids).

One set was placed in a chamber with 50% relative humidity/24ºC and the other set was placed in a chamber with 35ºC/32% relative humidity. Samples from each set were and measured on a Brookfield RVT viscometer at 25ºC.

Claims (9)

1. A method for creating a conformal coating on an object having covered areas, in which (a) a one-part, dual-cure, conformal coating composition is applied to the article, (b) the coating is exposed to radiation and (c) allowing atmospheric moisture to complete the curing of the coating, wherein the coating composition is solvent-free and (i) an isocyanate adduct having free isocyanate groups and free photopolymerizable ethylenically unsaturated groups, the ethylenically unsaturated groups accounting for 10 to 70% of the sum of all free functional groups of the adduct, (ii) a reactive (meth)acrylate diluent and (iii) optionally photoinitiator includes. 2. The process of claim 1 wherein (i) is the reaction product of an aliphatic isocyanate adduct and a hydroxyalkyl (meth)acrylate. 3. The process of claim 2, wherein the aliphatic isocyanate adduct is an aliphatic isocyanate trimer having an isocyanurate ring. 4. The process of claim 3, wherein the adduct is a trimerization product of hexamethylene diisocyanate. 5. A process according to any one of the preceding claims, wherein the free ethylenically unsaturated groups make up about 20% of the total number of free functional groups of the adduct. 6. A method for creating a contour-accurate coating on an object having shadow areas, in which (a) a one-part, dual-cure, conformal coating composition is applied to the article, (b) the coating is exposed to radiation and (c) allowing atmospheric moisture to complete the curing of the coating, wherein the coating composition is solvent-free and comprises aliphatic isocyanate trimer having an isocyanurate ring according to the preceding claim 3. 7. The process of claim 6, wherein the trimer is the trimerization product of hexamethylene diisocyanate. 8. A process according to claim 6 or 7, further comprising a reactive (meth)acrylate diluent and optionally photoinitiator. 9. Method according to one of the preceding claims, in which the object is a printed circuit board.